The invention relates to an instrument for plasma coagulation.
Such an instrument is known for example from the document EP-1293170, and in the following is explained with reference to FIG. 3.
In FIG. 3 a probe for coagulation of tissue by means of plasma, specifically by means of argon (APC), is shown as a longitudinal section through the end section of the probe, which during an operation projects out of the opening of the working channel of an endoscope. Argon gas flows through the lumen 11 of a probe body 10 and emerges from an outlet 12 of the probe body 10.
Within the lumen 11 of the probe body 10 is an ignition electrode 20 that comprises, at its end opposite to a tip 21, a helical section 6 coiled in such a way that the outer circumference of the helix 6 is apposed firmly to a tube wall 13 of the probe body 10, or is fixedly attached thereto. The end of the helical section 6 opposite to the ignition electrode 20 is connected by way of a crimp tubule 7 to a current conductor 25 for supplying a coagulation current. The ignition electrode 20 is situated in the interior of a tubule 14 that is made of ceramic and likewise is firmly pressed into the tube wall 13. The arrangement here is such that the ignition electrode 20 with its tip 21 is set back by a prespecified amount from the outlet 12 of the tube body 10, or of the tubule 14 situated therein. Furthermore, the ignition electrode 20 occupies a precisely coaxial position within the tubule 14, so that symmetrical relationships are produced when the argon that flows past the ignition electrode 20 and its tip 21 and then emerges through the outlet 12 is ionized by the coagulation current and thus, in the known manner, produces a plasma that coagulates the tissue to be treated.
The known arrangement raises many problems, with regard in particular to manufacture but also to the operation of the instrument, in particular long-term operation.
First, it is difficult to provide the ignition electrode 20 with a helical section 6 having sufficiently precise construction that when it is pressed into the probe body 10, the ignition electrode 20 and in particular the tip 21 are positioned exactly in the middle, i.e. are coaxial with the tubule 14. Another problem arises in that the distance separating the tip 21 from the outlet 12 of the tubule 14 depends on the depth to which the helix 6 has been pressed in. Even placing the helix 6 in direct contact with the back end of the tubule 14 cannot alleviate this problem, because pressing too firmly would cause a change in position and/or angle of the ignition electrode 20.
The connection between the current conductor 25 and the electrode 20 by way of the crimp tubule 7 requires much effort to produce, in which respect it should always be kept in mind that the lumen 11 has a very small diameter, in the case of probes a diameter in the lower mm range or even below that.
Another problem with the known instrument resides in the fact that in particular the helical section 6 and its transition to the straight ignition electrode 20 cause, firstly, a constriction of the space through which the inert gas flows, and furthermore in the transition region between the helical section 6 and the interior of the tubule an asymmetry is present, which results in nonuniform gas-flow relationships within the tubule 14. All these asymmetries and constrictions interfere with the plasma generation and in particular result in poor reproducibility of instruments manufactured in series, with regard to their ignition behavior.
In a thermal respect, as well, the known state of the art raises problems. That is, the plasma causes heating and even burning away of the ignition electrode 20, so that the burning behavior, and in particular the ignition behavior of the instrument, changes while it is in use, which in turn makes it necessary to readjust the coagulation current (and/or the amplitude of the applied HF voltage).